LD 11691 The present application is directed to electronic ballasts, and more particularly to a single stage High Intensity Discharge (HID) electronic ballast.
HID electronic ballasts have been gaining in popularity due to their efficiency and capability to increase the life of a lamp associated with the HID ballast. It is also known that HID electronic ballasts permit easy control of lumen output of the lamp when compared to other ballast types.
However, certain drawbacks have limited the implementation of HID ballasts. One of these drawbacks is related to acoustic resonance issues. Particularly, if a lamp, such as an HID lamp, is operated at a frequency in its acoustic resonant range, the life of that lamp will be reduced. HID ballasts now available commonly implement a low frequency square waveform output to the lamp to avoid the acoustic resonant due to the high frequency operation.
Another drawback is that existing HID ballasts are implemented in multiple independent stages. For example, in a three-stage ballast, the first stage is designed to convert an AC input signal to a DC output voltage. This conversion is commonly accomplished through the use of a power factor correction stage. Therefore, the first stage performs the conversion to a roughly regulated DC voltage and also corrects the circuit power factor. Correction of the power factor is intended to provide low Total Harmonic Distortion (THD), and high power factor to input line current. A second stage may be a buck converter stage used to regulate the output current. This second stage is directed to converting DC voltage to DC current output by controlling the duty cycle of the buck converter. A third stage is used to supply an AC current to the lamp. Commonly, lamps are designed to operate on the AC current. Therefore, the third stage acts to convert the DC current to an AC current. In one embodiment, the third stage may be implemented through the use of a full-bridge converter circuit. This third stage may be combined with an igniter circuit to start associated HID lamp, which will often require an approximately a 3 kv starting pulse in order to break down the gas within the lamp envelope.
As may be realized from the above discussion, ballasts implementing multiple stages require a large number of components, resulting in higher configuration and construction costs and an increase in the likelihood of component failures. These high costs, need for many components, and lack of reliability are additional factors why HID electronic ballasts are not implemented and used as widely as possible.
Attempts to address existing drawbacks have been made. One particular attempt is described in Maheshwari, et al., U.S. Pat. No. 5,932,976. This patent implements, as in the previous systems, a low frequency square waveform output. The stated innovation is the combination of a high frequency starting operation with a low frequency output. However, this patent still implements three stages, wherein a high number of components are used, resulting in lower reliability and high configuration and construction costs.
A second patent addressing HID ballasts, is to Beasley, U.S. Pat. No. 5,796,216. In the '216 patent, instead of the lamp being driven at a high frequency at start-up and being driven at a low frequency during steady state operation, the lamp is also operated at a high frequency during steady state operation, sufficient to avoid acoustic resonance frequency of the lamp. In design, the circuit of '216 implements a high power factor correction stage which converts AC voltage input to a DC voltage output. Then the input current is controlled to provide a low THD and a regulated output. Another stage is a half-bridge inverter circuit which converts the DC signal to an AC signal to drive the lamp with a series resonant circuit.
The high frequency achieved by the resonant circuit during the starting phase is turned into a third harmonic of the driver frequency. So during the starting phase, the series resonant parallel loaded circuit is unloaded and the resonant frequency is resonating on the third harmonic of the drive frequency. Therefore, for example, if the switching frequency is 100 kHz, then the frequency at the third harmonic is 300 kHz, and the output to the lamp will see is the 300 kHz.
Thus, U.S. Pat. No. '216, uses a third harmonic resonance for starting of the lamp, which is intended to reduce the stress in the inverter circuit, as well as stress to the inductor and to the half-bridge circuit. However, a drawback with this design is the circuit needs to be precisely tuned, which makes the manufacturing process a very complicated undertaking. Since, if the tuning of the circuit is not accomplished properly, it results in poor circuit performance and, thereby brings into question reliability issues. This circuit also is a multi-stage design bringing into issue component count, costs and reliability.